The popularity of alternative Web browser Firefox continues to rise at the expense of Microsoft's Internet Explorer, according to a new study.

From the beginning of December through mid-January, 4.78 percent of Internet surfers studied by online measurement company WebSideStory used the Mozilla Foundation's Firefox browser, a gain of 0.88 percentage points. At the same time, IE usage declined 0.7 percent to 92.7 percent, the firm reported. WebSideStory said IE use has declined from 96.7 percent since June.

The study measured market share by embedding sensors on major Web sites such as those of the Walt Disney Internet Group, Best Buy, Sony and Liz Claiborne. Previous studies from WebSideStory tested all operating systems, but the company said its Windows-only numbers are more accurate because new configurations in Apple Computer's Safari browser inadvertently skewed results. WebSideStory retrieves data from 30 million Internet users a day passing through its monitored sites. The company then takes a snapshot of two days and compares the growth.

Mozilla, an open-source software foundation formed by Netscape, launched Firefox 1.0 in November, after recording more than 8 million downloads of its test version. As downloads continued to surge, measurement firms such as WebSideStory and Dutch market researcher OneStat.com began releasing data tracking Firefox gains and IE declines. In December, OneStat reported that IE's market share had slipped to 88.9 percent, a figure Microsoft disputed.

Even though Microsoft maintains the dominance it has enjoyed since defeating Netscape Communications' browser in the mid-1990s, Firefox's snowballing popularity is raising eyebrows.

Much of Firefox's success rides on it simply not being IE. Firefox fans have praised the new browser's automatic pop-up blocking and tabbed browsing features, but security has been one of the big points. Viruses and spyware are often aimed at IE because of its near-total market dominance.

However, Firefox's popularity may eventually attract the attention of malicious code writers trying to exploit security holes. The recent discovery of a potentially damaging software flaw suggested the potential for Firefox attacks.

Since beginning its measurements last summer, WebSideStory has been cautious to draw any broad conclusions about Firefox's popularity. This time around, the company said many people are not only downloading Firefox, they're sticking with it and using it.

"We are finding that in many cases there's room for two browsers on the desktop, and in other cases people are completely switched over," said Geoff Johnston, an analyst at WebSideStory.